Photographic silver halide emulsions containing thioethers of saturated oxygen heterocycles as stabilizers and antifoggants



ilnited States Patent i PHOTOGRAPHIC SILVER HALIlDE EMULSIGNS CGNTAHNING THISETHERS OF ATURATED OXYGEN HETEROCYCLES AS STABILEZERS AND ANTEFGGGANTS Guenther H. Klinger, Binghamton, NFL, assignor to General Aniline & Film Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed Feb. 1, 1961, Ser. No. 86,280 6 Claims. (CI. 96-63) This invention relates to light sensitive silver halide photographic emulsions which contain as stabilizers and antifoggants thioethers of fully saturated oxygen-containmg heterocyclic ring systems having one nuclear oxygen atom and 4 to 5 carbon atoms in the ring. The invention also relates to methods of preparing such photographic emulsions and to photographic elements prepared therefrom.

It is known that silver halide photographic emulsions, when stored for any length of time, become spontaneously developa-ble without exposure to light. The degree of developability depends on the storage conditions. When photographic elements containing such excessively aged emulsions are exposed and developed, there is produced,

ieven in the unexposed areas of the plate, a certain quantity of developed silver. Optical opacity produced in this manner is commonly referred to as fog, and emulsions which gave rise to this condition are designated as fogged emulsions.

This photographic phenomenon has long engaged the attention of skilled artisans in the field; and numerous 3,%8,099 Patented Dec. 11, 1962 'ice tin undercoat. In this system, the stabilizers will slowly migrate into the silver halide layer during normal development. The object is to adjust dilfusion time to such a rate that stabilizers become available or difluse into the emulsion layer at about the time that the main development is consummated so that the stabilizers will arrive on the scene to retard fog development which as previously pointed out lags behind normal development. Although this system is sound in principle, it is difficult to reduce to practice, mainly because all known mercapto stabilizers diffuse very rapidly, even during the coating operations, and, consequently depress normal photographic development with concomitant reduction in usable density.

I have now discovered a way of utilizing the stabilizing action and fog inhibiting properties of mercapto compounds while at the same time eliminating the deleterious photographic side effects normally associated with these adjuncts, i.e., developer suppression in the exposed areas of a silver halide emulsion with concomitant reduction in photographic speed.

The use of these mercapto compounds to inhibit fog and to stabilize photographic emulsions while simultaneously maintaining the speed and sensitivity of said emulsion constitutes the objects and purposes of this invention.

Other objects and purposes will become apparent as the description proceeds.

reasons and theories have been enunciated to account for this peculiarity of photographic emulsions. For instance, it 1s generally known that conditions of excessive humidity durmg storage aggravate or accelerate the formation of fog. In fact, it is a common practice to actually test photographic emulsions by exposing them to excessive humidity and temperature in order to ascertain their tendency toward fog formation. Such information is, of course, very important since emulsions may be subjected to the aforedescribed conditions when used and stored in tropical climes. Photographic fog can also be produced by exposure to chemicals such as hydrogen, sulfide or other reactive sulfur gaseous compounds, including strongly reducing materials. Many of these materials are found in the atmosphere.

A large number of substances have been proposed as being effective in retarding or preventing fog when such entities are incorporated in light sensitive silver halide emulsions. These photographic adjuncts are commonly referred to as antifoggants or stabilizers, and are often compounds which have a mercapto function as the reactive group in their molecular structure. Although mercapto compounds may be effective in reducing or preventing fog, they almost always depress development rate in the exposed areas resulting in diminished speed. To overcome this dilemma, it has been suggested to employ a dual development technique based on the difference of developability of exposed versus fogged emulsion.

It is well known that during photographic development, fog generally develops at a lower rate than that of the usable density. This differential developability can be utilized by initially developing the photographic material in a developer which contains no stabilizers and as soon as fog development begins further development can be continued in a developer free of mercapto stabilizers. In this way, it is possible to gain higher usable density while maintaining a lower fog level. However, the use of two separate developing operations is impractical and tedious. A modification of the two stage developer technique locates non-diffusing mercapto stabilizers in a gela- I have now discovered that mercapto compounds, when employed in the form of their thioethers, particularly their tetrahydropyranthioethers, are excellent antifoggants and stabilizers for silver halide emulsions without the usual effect of diminishing photographic speed by retarding development in the exposed areas. Moreover, these mercapto ethersare effective in retarding speed loss during the aging of emulsions and are thus effective stabilizers as well as antifoggants.

Exemplary tetrahydropyranthioethers, the use of which is contemplated herein can be represented by the following formula:

it 2 I R 1 i wherein R represents an alkyl group of from 1 to 20 carbon atoms such as methyl, ethyl, propyl, butyl, amyl,

, matic hydrocarbon radical, etc., a heterocyclic ring system of the thiothiazole series, e.g., thiazole, 4-rnethy1thiazole, 4 phenylthiazole, S-methylthiazole, S-phenylthiazole, 4,5- dimethylthiazole, 4,5-diphenylthiazole, 4-(2-chloropheny1)thiazole, etc., those of the benzothiazole series, e.g., benzothiazole, 4-chlorobenzothiazole, 5-chlorobenzothi azole, 6-chlorobenzothiazole, 4-methy1benzothiazole, 5- rnethylbenzothiazole, S-bromobenzothiazole, 4-phenylbenzothiazole, S-phenylbenzothiazole, S-methoxybenzothiazole, 6-methoxybenzothiazole, 4-ethoxybenzothiazole, tetrahydrobenzothiazole, 6-hydroxybenzothiazole, etc., those of the naphthothiazole series, e.g., fi-naphthothiazole, a-naphthothiazole, 5-methoxy-fi-naphthothiazole, S-ethoxy-fl-naphthothiazole, S-methoxy-x-naphthothiazole, etc., those of the benzoxazole series, e.g., benzoxazole S-chlorobenzoxazole, S-methylbenzoxazole, S-phenylbenzoxazole, 6-methylbenzoxazole, 4,5-dimethylbenzoxazole, S-chlorobenzoxazole, 6-methoxybenzoxazcle, 6-hydroxybenzoxazole, etc., those of the benzoselenazole series, e.g., benzoselenazole, S-chlorobenzoselenazole, S-methoxybenzoselenazole, etc., those of the quinoline series, e.g., quinoline,

spasms 1\ 0 j 30 61-133 ll.

grasses The tetrahydropyran thioethers used in practicing the invention can be prepared by condensing the requisite Z-mercapto compound, e.g., a mercaptoazole with a dihydropyran in the presence of an acidic condensing agent such as p-toluene sulfonic acid. The reaction is conveniently carried out by refluxing the components in a suitable solvent after which the desired tetrahydropyran thioether is isolated by the well known methods of organic chemistry, such as crystallization or distillation.

An unusual characteristic of the tetrahydropyran thio ethers contemplated herein is their cleavage in the presence of silver ion with regeneration of compounds containing free mercapto groups. It is this special property of tetrahydropyran thioethers which is considered to account for their efiiciency as stabilizers and antifoggants. However, any explanation in this connection is ofiered only as a theory and is not to be taken as limiting the invention to this particular mode of operation. It is possible that other mechanisms would provide a more complete interpretation of the behavior of the aforesaid entities.

Most photographic developers contain silver halide solvents, as for example sulfite. During the development process, small amounts of silver ions are in the solution due to the action of the aforementioned silver halide solvents. These silver ions are believed to split the tetrahydropyran ethers and to provide a molecule with a free mercapto group. At about the same time, chemical development of exposed grains begins to take place. There is no indication that the freshly generated free mercapto compound interferes with normal development as evidenced by an exceedingly low degree of fog density. Apparently the free mercapto compound is liberated at the optimum stage of development thus enabling it to exert maximum fog inhibiting power. It is not, however, formed or generated before normal development has proceeded to the optimum value. In other words, it is possible to harness the chemical reactions of development to regulate the production or generation of mercapto compound, in situ, to procure optimum antifogging prop: erties associated with such a compound while, at the same time, eliminating the usual speed loss caused by the presence of tree mercapto additives. It will be appreciated that the theory or mechanism as above set forth is tentative and may not be correct in all its aspects, and may possibly be refined or elaborated subsequently. However, it is submitted the concept of freeing a mereapto compound so as to approximately coincide with the development action results in heretofore unobtainable advantages and the present invention thus represents a concept not previously contemplated by the prior art.

The aforementioned compounds can be utilized in various ways to secure beneficial eliects and fog reduction. For instance, the components can be incorporated in the silver halide emulsions as ripening finals or as coating finals. Ripening finals are added during the ripen? ing or the sensitivity increasing stage of the emulsionmaking process. These additions may be made before, during or after the double decomposition reaction between the soluble silver salt such as silver nitrate and the soluble halide such as potassium bromide. The latter reaction is, of course, carried out in the presence of a colloid carrier. By colloid carrier is intended the medium in which is contained the light sensitive silver halide salts. Typical examples of these carriers include gelatin, PVA, solubilized casein, carboxymethyl cellulose, albumin and the like.

Coating finals are customarily added to the emulsion just prior to coating on a suitable support such as glass, paper or film at a time when the emulsion is approaching its maximum sensitivity.

When incorporated directly in a silver halide emulsion, the tetrahydropyran thioethers of the type described herein are preferably used in quantities ranging from 2 to 30 mg. per mole of silver bromide. It will, of course, be appreciated that the concentration will vary with various types of emulsion, and it may be desirable to determine the optimum concentration from case to case. In some instances, it is advantageous to apply the antifoggants and stabilizers in adjacent layers, e.g., in a sepa rate undercoating layer or in the antiabrasion gelatin surface. Because of the physical separation between the silver halide emulsion layer and a separate undercoating layer, it is recommended that correspondingly larger quantities of antifo-ggant beused when it is contained in undercoating surface. In some cases, it may be desirable to utilize the antifoggants in one or all of the processing baths or in various preand postbaths. Those skilled in the art will be in a position to ascertain the particular location of the antifoggants for a given situation.

According to the invention, the antifoggants and stabilizers can be advantageously employed in connection with any type of photographic emulsion, e.g., non-sensitized, orthochromatic, panchromatic, X-ray emulsions, paper emulsions, color emulsions or the like. it has also been established that these compounds are usable in combination with other photographic adjuncts as exemplified by metal and noble sensitizers and in combination with a hydroxy polyphenoxy derivative of the type obtained by reacting ethylene oxide with an alcohol phenol amine or the like, as described in U.S.P. 1,970,578.

The invention is illustrated by the following examples, but it is to be understood that the invention is not to be restricted thereto.

EXAMPLE I A silver halide emulsion in gelatin containing 4% silver iodide and 96% silver bromide was prepared in a conventional manner and brought up to its maximum light sensitivity and then readied for coating. Finals were added, such as sensitizing dyes, hardening agents, or any various other photographic adjuncts commonly used in the art. The emulsion was divided into portions and to each was added varying quantities of the tetrahydropyran thioether of Z-mercaptobenzothiazole and dihydro- 7 pyran. The emulsion samples contained about .4 mole of silver halide. The so-prepared samples were coated on a suitable cellulose ester base and dried. Samples of other 1 /2 hours. The reaction mixture was cooled and washed Wlll'l sodium bicarbonate solution and with water. The solution was dried with Na SO filtered and evapthese film coatings were exposed in a Type 113 sensitqfllorated. The resulting syrup crystallized upon scratching flier developed 111 a develc'pef DI the following 5 with acetone. The crystals were recrystallized from n- COmPOSItIOH- G propanol. Yield 14 g. yellow crystals=52%. Cale.

mm C I NS 0: Metol 1.5 13 2 Sodium sulfite (anhydrous) 45.0 C 1 r I q a 0., cunt, odlum Plsulfite percent percent Hydroqurnone 3.0 Sodium carbonate (monohydrated) 6.0 58157 57.2 Potassium bromide 0.80 g {3- Water to make 1.0 liter.

In Table I are compared various development times 15 m of the coatings containing the tetrahydropyran thioether blmllafly fifi S 5-mercapto Z-phenyltetrazole of Z-mercaptobenzothiazole on the one hand versus a The Yield was 57 'Y fwm typical antlfoggant on the other, that is, 2-phenylmereap- MBOH- M3 Q (3310- r2 14 4 tobenzothiazole. 262.32.

TABLE I All tests developed in the developer of Example I Fresh tests Aging tests 583g; Addenda 3 minutes 6 minutes 12 minutes Tank oven 1 6 Pouch oven 1 6 of test development development development minutes developminutes development ment Speed at Speed at Speed at Speed at Speed at Fog 0.2 above Fog 0.2 above Fog 0.2 above Fog 0.2 above Fog 0.2above fog tog fog fog f p 0.13 65 0.21 91 0. 45 mg. of TPT per mole Ag Br 0. 9 0.15 91 0.35 mg. of TPT per mole Ag Br 0. 9 64 0.13 110 0.30 mg. of 'IPT per mole Ag Br 0.9 58 0.11 89 O. 26 0 mg. of TPT per mole Ag Br 0. 9 48 0. 9 78 0.16 5 mg. or PM'I per mole Ag Br 0. 8 23 0. 8 46 0.10 n g. of PMT per mole Ag Br 0.6 12 0.8 26 0.9 mg. of 'IPT per mole Ag Br 0.12 88 mg. of PM'I per mole Ag Br 0.9 29

1 Tank oven aging: The film was exposed to relative humidity at 35 C. for 4 days. 2 Pouch oven aging: The film was sealed into a plastic envelope at a relative hurndity of 53% and kept at 49 C. for 6 days.

Norm-TF1 equals tetrahydropyran ether of 2-mereaptobenzothiazole: PMI equals phenylmercaptotetrazole.

As can readily be ascertained, the tetrahydropyranether while functioning as an efficient antifoggant agent does not suppress or cut down on the speed, whereas the phenylmercapto benzothiazole, although capable of retarding fog, also cuts prohibitively into the sensitivity and speed. From the incubation information, i.e., tank oven and pouch oven tests, it is at once evident that the tetrahydropyranethers are excellent stabilizing agents for retarding the degradation of photographic emulsions under conditions of excess humidity and temperature. Very little stabilizing effect is exhibited by the 2-phenylmercaptobenzothiazole which, as previously pointed out, is a typical antifoggant proposed by the prior art.

EXAMPLE II The procedure of Example I was repeated with the exception that the tetrahydropyran thioether of Example I was replaced by the corresponding derivative of phenylmercaptotetrazole. Results closely paralleled those of the earlier example.

Preparation of Tetra/1ydropyranthioethers Cale, Found.

percent percent 0 54. 94 55. 34 H 5. 38 5. 56 N 21. 36 21. 31 R 12. 22 12. 94

I have also ascertained that the thioethers produced from 2,3-dihydrofurane are likewise effective as antifoggants and/or stabilizers and in this respect resemble the previously described thioethers derived from dihydropyrane. The synthesis of 2,3-dihydropyran in excellent yields is described in the chemical literature and in this connection reference is made to Angewandte Chemie, November 21, 1960, page 864. In forming the thioethers from 2,3-hydrofurane the same procedure is employed as utilized in the preparation of the corresponding derivatives of the dihydropyrane.

I claim:

1. A light sensitive photographic element comprising a base having thereon a light sensitive silver halide emulsion layer said element containing as an antifoggant a compound of the following formula:

wherein R is selected from the class consisting of an alkyl group of from 1 to 20 carbon atoms, a benzyl group, a phenethyl group, a naphthylmethyl group, a cyclopentyl group, a cyclohexyl group, an aryl group selected from the class consisting of mono and bicyclic aromatic hydrocarbon radicals, a heterocyclic nucleus of the thiazole series, benzoxazole series, benzoselenazole series, quinoline series, benzimidazole series, tetrazole series and pyridine series and R and R are selected from the class consisting of hydrogen and an alkyl group from 1 to 20 carbon atoms.

2. The article as defined in claim 1 wherein said tetrahydropyran thioether is located in the silver halide emulsion.

3. The article as defined in claim 1 wherein said tetrahydropyran thioether is located in a layer adjacent to said silver halide emulsion.

4. A light sensitive photographic element comprising a base having thereon a light sensitive silver halide emulsion layer, said element containing as an antifoggant and stabilizer a compound of the following formula:

5. The process of minimizing and preventing fog in a light sensitive silver halide material comprising a base having a light sensitive silver halide emulsion thereon which comprises exposing said emulsion to light and developing the same in the presence of an antifoggant compound of the following formula:

5 re i O SR wherein R is selected from the class consisting of an alkyl group of from 1 to 20 carbon atoms, a benzyl group, a phenethyl group, a naphthylmethyl group, a cyclopentyl group, a cyclohexyl group, an aryl group selected from the class consisting of mono and bicyclic aromatic hydrocarbon radicals, a heterocyclic nucleus of the thiazole series, benzoxazole series, benzoselenazole series, quinoline series, benzimidazole series, tetrazole series, and pyridine series and R and R are selected from the class consisting of hydrogen and an alkyl group of from 1 to 20 carbon atoms.

6. The process as defined in claim 5 wherein the antifoggant and stabilizer is represented by the following formula:

References Cited in the file of this patent UNITED STATES PATENTS 2,855,301 Larson et a1. Oct. 7, 1958 

5. THE PROCESS OF MINIMIZINMG AND PREVENTING FOG IN A LIGHT SENSITIVE SILVER HALIDE MATERIAL COMPRISING A BASE HAVING A LIGHT SENSITIVE SILVER HALIDE EMULSION THEREON WHICH COMPRISES EXPOSING SAID EMULSION TO LIGHT AND DEVELOPING THE SAME IN THE PROCESS OF AN ANTIFOGANT COMPOUND OF THE FOLLOWING FORMULA: 